This invention relates to a magnetic brush developing device for electrostatographic apparatus.
In known magnetic brush developing devices, it is common to develop an electrostatic latent image formed on a rotating or moving electrostatic imagebearing member in the form of a belt, sheet, or cylinder, by applying developing material to the image-bearing member. In magnetic brush developing arrangements, developer material is formed into a brush-like configuration on a non-magnetic, rotating, developing roll inside which are located magnetic field defining members. During development, the outer surface of the belt-like or cylindrically shaped electrostatic image-bearing member approaches or separates from the developing roll due to irregularity of its rotation radius or surface irregularities. This will be hereinafter referred to simply as "run-out" phenomenon and is characterized by a varying rather than constant spacing between the developing roll and the image bearing member during the development process.
Referring to FIG. 1 there is shown a known magnetic brush developing device in which an electrostatic image-bearing member 1 (in the form of a photosensitive drum) rotates in the direction of the arrow D. A magnetic brush developing device 2 is arranged in a suitable position around the electrostatic image-bearing member 1 and includes a housing or enclosure 3 having therein a developing roll 4 and a feed roll 5 rotating in the directions of arrows A and B, respectively. The developer 6 provided at the bottom or sump portion of the enclosure 3 is transferred to a developing zone C by the rotation and magnetic force of the developing roll 4 having therein a magnetic field developing means in the form of stationary magnets 41. An electrostatic latent image is formed on the photosensitive drum surface by well known charging and exposure steps (not shown) of the electrophotographic reproduction process and presented for development at the developing position C. A developer discharge-regulating plate or trimmer bar 7 is usually provided to control the amount of developer material 6 passing from the developer housing to the developing zone C via the roll 4. The trimmer bar usually extends across the width of the development enclosure 3 parallel to and spaced from the roll 4 in an area upstream of or adjacent the area of closest proximity between the roll 4 and surface 1. The clearance between the bar 7 and the roll 4 is preset and fixed to provide a preselected height of developer material reaching the development zone C.
However, the developing conditions including the clearance between the photosensitive drum 1 and the developing roll 4 invariably change or vary due to the run-out phenomenon and it is thus difficult to achieve an image with a constant image density. In addition, when the magnetic brush developing device is disassembled from the duplicating machine for maintenance of the device or upon occurrence of a problem, and later reassembled, the developing conditions which had been optimally adjusted are undesirably changed. The developed image which is produced under such conditions will be changed in image density or will be stained at background portions, it being difficult to obtain stable images of good quality.
In order to overcome the above difficulty, there has been proposed a magnetic brush developing device in which the clearance between the photosensitive drum 1 and the developing roll 4 is maintained constant by moving the magnetic brush developing roll in accordance with the "run-out" of the photosensitive drum 1. An arrangement of this general type is disclosed in U.S. Pat. No. 3,948,217. However, the magnetic brush developing device of the above-mentioned type has the disadvantage that it requires the quick movement of one or more magnetic brush rolls of significant weight and volume. In addition, the electrostatic latent image on the photosensitive drum 1 is adversely influenced due to vibrations of the developing device, since the photosensitive drum 1 is indirectly carried from the same mounting frame as magnetic brush developing device.